ASoC: imx-ssi.c: add new choices to platform configuration

* introduce 3 new flags to allow a more detailed configuration
of the SSI link :
	IMX_SSI_NET : enable Network Mode
	IMX_SSI_SYN : enable Synchronous Mode
	IMX_SSI_USE_I2S_SLAVE : enable I2S Slave Mode
* new platform can use these settings without breaking actual
platforms.

Signed-off-by: Eric Bénard <eric@eukrea.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>